I have a Xantia and sometimes if the wiring beneath the front seats gets disturbed this happens, as the wires have push together connectors. Make sure the ignition is off and you are not directly in front of the airbag if you fiddle about. If you want to disconnect them and re-connect them disconnect the battery and leave for at least 10 mins. Then undo them and reconnect them. Redo the battery. Then standing outside of the car with the window open, turn the ignition on with no part of your body in front of the airbag.
If this doesn't clear it, you may have a faulty sensor unit which I understand is a pretty expensive thing to replace.
With mine it is usually under the drivers seat where the trouble occurs.